Re: [PATCH v5 4/6] sched/isolation: Force housekeeping if isolcpus and nohz_full don't leave any

[Gabriele Monaco <gmonaco@redhat.com>]

On Tue, 2025-05-20 at 14:02 +0200, Frederic Weisbecker wrote:
> Le Thu, May 08, 2025 at 04:53:24PM +0200, Gabriele Monaco a écrit :
> > Currently the user can set up isolcpus and nohz_full in such a way
> > that
> > leaves no housekeeping CPU (i.e. no CPU that is neither domain
> > isolated
> > nor nohz full). This can be a problem for other subsystems (e.g.
> > the
> > timer wheel imgration).
> > 
> > Prevent this configuration by setting the boot CPU as housekeeping
> > if
> > the union of isolcpus and nohz_full covers all CPUs. In a similar
> > fashion as it already happens if either of them covers all CPUs.
> > 
> > Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
> > ---
> >  include/linux/tick.h     |  2 ++
> >  kernel/sched/isolation.c | 20 ++++++++++++++++++++
> >  kernel/time/tick-sched.c |  7 +++++++
> >  3 files changed, 29 insertions(+)
> > 
> > diff --git a/include/linux/tick.h b/include/linux/tick.h
> > index b8ddc8e631a3..0b32c0bd3512 100644
> > --- a/include/linux/tick.h
> > +++ b/include/linux/tick.h
> > @@ -278,6 +278,7 @@ static inline void tick_dep_clear_signal(struct
> > signal_struct *signal,
> >  extern void tick_nohz_full_kick_cpu(int cpu);
> >  extern void __tick_nohz_task_switch(void);
> >  extern void __init tick_nohz_full_setup(cpumask_var_t cpumask);
> > +extern void __init tick_nohz_full_clear_cpu(unsigned int cpu);
> >  #else
> >  static inline bool tick_nohz_full_enabled(void) { return false; }
> >  static inline bool tick_nohz_full_cpu(int cpu) { return false; }
> > @@ -304,6 +305,7 @@ static inline void tick_dep_clear_signal(struct
> > signal_struct *signal,
> >  static inline void tick_nohz_full_kick_cpu(int cpu) { }
> >  static inline void __tick_nohz_task_switch(void) { }
> >  static inline void tick_nohz_full_setup(cpumask_var_t cpumask) { }
> > +static inline void tick_nohz_full_clear_cpu(unsigned int cpu) { }
> >  #endif
> >  
> >  static inline void tick_nohz_task_switch(void)
> > diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
> > index 81bc8b329ef1..27b65b401534 100644
> > --- a/kernel/sched/isolation.c
> > +++ b/kernel/sched/isolation.c
> > @@ -165,6 +165,26 @@ static int __init housekeeping_setup(char
> > *str, unsigned long flags)
> >  			}
> >  		}
> >  
> > +		/* Check in combination with the previously set
> > cpumask */
> > +		type = find_first_bit(&housekeeping.flags,
> > HK_TYPE_MAX);
> > +		first_cpu =
> > cpumask_first_and_and(cpu_present_mask,
> > +						 
> > housekeeping_staging,
> > +						 
> > housekeeping.cpumasks[type]);
> > +		if (first_cpu >= nr_cpu_ids || first_cpu >=
> > setup_max_cpus) {
> > +			pr_warn("Housekeeping: must include one
> > present CPU neither "
> > +				"in nohz_full= nor in isolcpus=,
> > using boot CPU:%d\n",
> > +				smp_processor_id());
> 
> I wouldn't even bother recovering:
> 
> pr_warn("Housekeeping: must include one present CPU neither in
> nohz_full= nor in
> isolcpus=\n ignoring setting %lx", flags);
> 
> goto free_housekeeping_staging;

Yeah good point, that would simplify things with the tick CPU.

Thanks,
Gabriele